libgtk2: replace dependency on X.org server by dependency on libraries

libgtk2 is a X client library, so it doesn't make sense for it to
depend on the X.org server. Instead, it should depend on the X client
libraries.

This patch therefore replaces the dependency on the X server by a
dependency on libX11, libXext, libXrender and fontconfig, that are the
mandatory requirements to build the X backend of Gtk.

[Peter: don't add an empty line before gtk demo help text]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>